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New Scientist Live

Steampunk chip takes the heat

By Paul Marks

Steampunk, the reimagining of modern day technology through a Victorian perspective, has found an unlikely follower in the US Defense Advanced Research Projects Agency (DARPA). A DARPA-funded project has reinvented a type of logic gate in the style of Victorian inventor Charles Babbage – not for aesthetic reasons, but because the retro device works at temperatures too high for conventional transistors. It could therefore find uses in, say, jet and rocket engine electronics.

Babbage famously designed mechanical computers through which data would circulate as steam-driven pistons turned cogs and levers. His unwieldy contraptions were superseded by electronic computers, through which data is transmitted via vast arrays of transistors in the form of varying voltages.

In a transistor, the voltage applied to one of the terminals, the gate, determines whether a current flows through it. But above 250 °C, the device becomes so awash with thermally generated electrons – even when it is supposedly off – that the voltage leaks through the gate to render the device useless. Even silicon carbide, the semiconductor material hardiest against heat, doesn’t remedy the situation.

Modern Morse key

That prompted Te-Hao Lee’s team at Case Western Reserve University in Cleveland, Ohio, to consider returning to mechanical logic. His team has developed a mechanical version of an inverter – the building block used to construct many types of logic gate, which themselves are a fundamental component of digital circuitry within computers. The device uses an arrangement of nanoscale levers instead of transistors. Like a telegraph operator’s Morse key, these levers physically make and break contact to pass or block currents.

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Application of a voltage makes the levers move under electrostatic attraction. At 550 °C Lee’s team managed to get the inverter to switch on and off 500,000 times a second &ndash performing a computation with each cycle. The faster the switching speed, the zippier the computing. Lee predicts that switching speeds of a billion times a second (1 gigahertz) are possible. That might not sound fast by the standards of desktop PCs, which often run at speeds in excess of 2.5 gigahertz, but for control system applications it’s more than adequate. The leakage current was too small to measure, showing the researchers had overcome the current loss issue in transistors.

Lever logic

They are not there yet, though. Some levers have been melting and breaking after 2 billion cycles. “We are not sure why yet. But we think there is a temperature-related electrical spike occurring during the switching operation,” says Lee. He is confident of fixing it, however, and going on to develop other types of lever-based logic gates.

We can expect more such developments, says David Wright, an electronics engineer at the University of Exeter, UK. “Mechanical memory and logic devices are being developed by several groups worldwide.”